The present invention relates to a voltage regulator for a microcomputer system.
Microcomputers are now being applied to various types of machine control. In an internal combustion engine with a fuel injection system, for example, a microcomputer is used for calculating the amount of fuel to be supplied to the engine in accordance with a plurality of parameters such as the rotational speed of the engine and the intake air amount.
In that case, the amount of the fuel must be repeatedly calculated in a short period of time by means of the microcomputer so as to enable precise control of the fuel supply.
Therefore, a number of calculation cycles are executed, in the microcomputer, in accordance with a program stored in a memory which forms a loop of steps for each cycle of calculation.
This type of microcomputer is provided with a reset circuit for automatically resetting to an initial portion of the program steps upon occurrence of so-called "lost executable control", that is, a state in which the program is not correctly executed upon initial application of voltage to the microcomputer by a power supply. The reset circuit produces, immediately after application, or throw-in of voltage to the microcomputer by the power supply, a low level reset signal (logic zero) having a predetermined duration for example.
The reset signal is also produced when the reset circuit detects an abnormal generation of a run signal which is normally generated upon completion of each program cycle.
The thus produced reset signal is applied to a reset terminal of a microprocessor unit.
In the case of the conventional arrangement of the microcomputer system provided with the reset circuit mentioned above, however, there is a problem that the reset operation is not effected if the magnitude of the voltage applied to the microprocessor unit supplied from a voltage regulator circuit is not sufficiently high at the time when the reset signal is applied to the reset terminal of the microprocessor unit.